We received quite a few complaints about last week’s Casual Fridays study, most of them centered around our scientifically inaccurate eye exam. In our defense, the Snellen chart is only designed to be a rough measure of visual acuity. General practitioners use it as a first-pass to determine if patients should be referred to eye doctors or optometrists, who always use additional tests beyond the Snellen chart to determine prescriptions.

We’re not prescribing eyeglasses, just trying to get a rough sense of respondents’ vision. We thought giving a simple test would be easier than asking folks to try to remember or look up their prescriptions. However, our test would have been better if, as commenters pointed out, we had measured each eye separately, we had figured out a way to calibrate the display so it appeared the same size on all respondents’ screens, and if we could have delayed the display so there was less temptation to “cheat” by looking at the letters before removing eyeglasses or moving to the proper distance.

That said, despite the crudeness of our measure, we did obtain some significant results. First off, the 53 respondents who couldn’t read the top line of the chart (and are thus, like me, legally blind when not wearing corrective lenses) were unanimous in indicating that they wear their glasses or contacts “every waking moment” (though we are probably fudging this a bit — I don’t, for example, wear my glasses in the shower, where I generally make it a point to be awake).

But what of the others, who retain a reasonable ability to see things without correction? Here’s the breakdown of the people who said they wore corrective lenses “every waking moment.”

As you can see, apart from a curious three respondents who claim they can see line 4 perfectly from 10 feet (giving them better than “perfect” vision), as vision gets better, people tend to wear corrective lenses less (I suspect the three outliers misread the question).

Now let’s divide the responses into groups corresponding to their level of vision impairment. We’ll eliminate those who could see line 4, since that’s nearly perfect vision, as well as those who could see no better than line 70. We’ll lump lines 7 through 20 together, and 30 through 60 together.

If you have good to moderate vision, your most likely response is that you don’t have corrective lenses at all. With fair to poor vision, about two thirds of respondents wear corrective lenses all the time, with another 14 percent wearing them 75 percent of the time. Just 14.8 percent of the “good to moderate” group wears corrective lenses all the time. The times this group uses correction tend to be reading and driving.

Why don’t more people with good to moderate vision use correction? There are few consistent responses. Nearly thirty percent of this group claims to have perfect vision. Others say they’re uncomfortable, or they can do everything they need to. Others are more blunt: “too lazy to go and buy,” “I’m vain,” or “they’re always smudged, so they don’t help that much.”

Most people with fair to poor vision indicate that the only time they don’t use correction is for reading; otherwise, they use their glasses/contacts.

One respondent cited a common myth about eyeglasses: they feared that their vision would deteriorate more rapidly if they wore correction. There is no evidence to support this belief (nor, based on a fairly thorough search of the internet, does there appear to be evidence that not wearing glasses when you have an impairment accelerate deteriorating vision).

So where does our son Jim fall on this scale? Remember, our initial motivation for this particular study was our lack of success getting him to wear his glasses. He can read line 10 from 10 feet away, putting him solidly in the middle of the good-to-moderate group. Like most of the others in this group, he doesn’t see much reason to wear glasses, and so he doesn’t.